The invention relates to a method and to an apparatus for interferometrically determining a deviation of an actual shape of an effective reflection surface of a test object from a desired shape of the effective reflection surface. Furthermore, the invention relates to a diffractive optical element for this type of apparatus.
With interferometric systems for measuring a surface of an aspherical optical element known from the prior art, a plane wave produced by a laser is split into a reference beam and a measuring beam. The measuring beam is thereupon converted by means of a focussing lens and a CGH (computer-generated hologram) into a wave which corresponds to the desired shape of the surface to be tested. The wave is thereupon reflected by the surface to be measured. The wave front of the reflected wave contains the deviations of the measured surface from the desired shape. The reflected wave is guided back in the optical path of the incoming measuring beam through the CGH and the focussing lens. The plane wave thus produced is finally reflected away from the optical path of the input wave by means of a reflecting element and overlaid with the reference beam. An interference pattern is thus produced which is focussed onto a recording plane of a camera by means of a focussing lens. Deviations of the shape of the measured surface from a desired shape of the surface are determined from the recorded interference pattern.
However, the previously known interferometric systems themselves cause substantial deviations or errors in the wave fronts of the waves passing through them. In order to determine the deviation of the shape of the test object from the desired shape from the recorded interference pattern with a high level of precision, extensive calibration steps are therefore taken with the interferometric arrangements from the prior art. Here, wave fronts are measured and evaluated in a plurality of rotational positions of the test object. With the aid of this type of calibration measures interferometer errors can be calculated out of the measurement results. However, the test object must have a rotationally symmetrical surface for this purpose. This means that highly precise measurement of the surface is only possible for test objects with rotationally symmetrical surfaces with the aforementioned interferometric systems.